The maples and willow not only create pretty dense shade, but they've also got shallow, dense root systems that will compete with your plantings for space, moisture, and nutrients. But the morning sun and good soil you describe make it sound like a habitable garden spot for lots of plants, so go for it. Because you've got 8 feet across the top, you might want to plant a few shrubs like Hydrangeas. Perennials you might plant include Hostas, Corydalis, Pulmonaria, Brunnera, and Sweet Woodruff. You might also tuck in a few shade-loving groundcovers like Periwinkle (Vinca major), Pachysandra, Lily of the Valley, or Ajuga.